1. Field of the invention:
This invention relates to a method of improving the catalytic performance (i.e., sustaining and/or regenerating catalytic activity) of a carbon-supported, rhenium-containing catalyst and, in particular, a hydrogenation catalyst comprising a composite of palladium or ruthenium in combination with rhenium on a support of activated carbon. More specifically but not by way of limitation, this invention pertains to depositing and/or replenishing rhenium metal on bimetallic palladium and rhenium or ruthenium and rhenium on carbon support (i.e., Pd,Re/C or Ru,Re/C, respectively) catalysts or catalyst precursor by depositing fresh fmely divided rhenium metal on these catalysts during hydrogenation of maleic acid to tetrahydrofuran and/or 1,4-butanediol.
2. Description of the related art:
Supported rhenium containing catalysts are well known in the art and are useful in a number of important processes. Such catalysts include: palladium, rhenium/carbon catalysts and ruthenium, rhenium/carbon catalysts used in the hydrogenation of maleic acid to tetrahydrofuran; platinum; rhenium/alumina catalysts used in reforming of oil to gasoline; and rhenium on alumina catalyst used in olefin metathesis. During use of such catalysts, the catalytic activity of the catalyst can decrease over time as the rhenium component of the catalyst assumes a catalytically less active form.
There are many patents relating to the art of converting maleic acid or maleic anhydride or a variety of hydrogenatable precursors such as fumaric acid, succinic acid (SAC), maleic acid (MAC), dimethyl succinate, gamma-butyrolactone (GBL) or mixtures thereof to tetrahydrofuran (THF) and 1,4-butanediol (BDO) by catalytic hydrogenation. Of particular importance, U.S. Pat. No. 4,609,636 describes the preparation and use of a catalyst composite comprising palladium and rhenium on a carbon support for manufacture of THF, BDO or mixtures thereof from a variety of hydrogenatable precursors. This patent also cites patents that attempt to maximize product yields. Similarly, U.S. Pat. No. 5,418,952 describes the preparation and use of a catalyst composite comprising ruthenium and rhenium on a carbon support for the same use.
The catalysts of U.S. Pat. No. 4,609,636 and U.S. Pat. No. 4,550,185 are composites of palladium and rhenium on a carbon support, the composite comprising about 0.5% to 10% palladium and about 1% to 10% of rhenium by total weight, the palladium being present in the form of crystallites having an average size of about 10 nm to 25 nm and the rhenium being in the form of a highly dispersed phase of crystallites having an average size of less than about 2.5 nm. These catalysts are closely related to the catalyst of the present invention .
Also pertinent is a report by H. Smith Broadbent reported in Annals of the New York Academy of Sciences, 145 (1) (1967) (pages 68 through 71), which is a comprehensive study of "Rhenium and Its Compounds as Hydrogenation Catalysts". Broadbent teaches (page 62, last paragraph) in-situ preparation of an insoluble rhenium black by addition of rhenium heptoxide to a reducible organic substrate, with or without added solvent, followed by a hydrogenation step. Broadbent indicates the rhenium black is some form of Re(II), probably a hydrated monoxide. This catalyst, although less active than nickel or platinum metal hydrogenation catalysts for hydrogenation of most compounds, was judged superior to all other catalysts except, perhaps, other oxides of rhenium, for hydrogenation of carboxylic acids.